Technical Field
The invention relates generally to precision position or displacement measurement instruments, and more particularly to an encoder configuration with signal processing which is resistant to errors that may be associated with a contaminated or defective portion of a scale.
Description of the Related Art
Optical position encoders determine the displacement of a readhead relative to a scale that includes a pattern that is detected by the readhead. Typically, position encoders employ a scale that includes at least one scale track that has a periodic pattern, and the signals arising from that scale track are periodic as a function of displacement or position of the readhead along the scale track. Absolute type position encoders may use multiple scale tracks to provide a unique combination of signals at each position along an absolute scale.
Optical encoders may utilize incremental or absolute position scale structures. An incremental position scale structure allows the displacement of a readhead relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. Such encoders are suitable for certain applications, particularly those where line power is available. In low power consumption applications (e.g., battery powered gauges and the like), it is more desirable to use absolute position scale structures. Absolute position scale structures provide a unique output signal, or combination of signals, at each position along a scale, and therefore allow various power conservation schemes. U.S. Pat. Nos. 3,882,482; 5,965,879; 5,279,044; 5,886,519; 5,237,391; 5,442,166; 4,964,727; 4,414,754; 4,109,389; 5,773,820; and 5,010,655 disclose various encoder configurations and/or signal processing techniques relevant to absolute position encoders, and are hereby incorporated herein by reference in their entirety.
Some encoder configurations realize certain advantages by utilizing an illumination source light diffraction grating in an illumination portion of the encoder configuration. U.S. Pat. Nos. 8,941,052; 9,018,578; 9,029,757; and 9,080,899, each of which is hereby incorporated herein by reference in its entirety, disclose such encoder configurations. Some of the configurations disclosed in these patents may also be characterized as utilizing super resolution moiré imaging.
In various applications, scale manufacturing defects or contaminants such as dust or oils on a scale track may disturb the pattern detected by the readhead, creating errors in the resulting position or displacement measurements. In general, the size of errors due to a defect or contamination may depend on factors such as the size of the defect or contamination, the wavelength of the periodic pattern on the scale, the size of the readhead detector area, the relationship between these sizes, and the like. A variety of methods are known for responding to abnormal signals in an encoder. Almost all such methods are based on disabling the encoder signals, or providing an “error signal” to warn the user, or adjusting a light source intensity to boost low signals, or the like. However, such methods do not provide a means of continuing accurate measurement operations despite the abnormal signals that arise from certain types of scale defects or contamination. Therefore, these methods have limited utility. One known method that does mitigate the effects of scale contaminants or defects on measurement accuracy is disclosed in Japanese Patent Application JP2003-065803 (the '803 Application). The '803 Application teaches a method wherein two or more photo detectors output periodic signals having the same phase, which are each input to respective signal stability judging means. The signal stability judging means only outputs signals that are judged to be “normal,” and “normal” signals are combined as the basis for position measurement. Signals that are “abnormal” are excluded from position measurement calculations. However, the methods of judging “normal” and “abnormal” signals disclosed in the '803 Application have certain disadvantages that limit the utility of the teachings of the '803 Application.
U.S. Pat. No. 8,493,572 (the '572 patent) discloses a contamination and defect resistant optical encoder configuration which provides a means to select signals from photodetector elements which are not subject to contamination. However, the '572 patent relies on complex signal processing that may be less desirable in some applications.
Improved methods for providing accurate measurement operations that avoid or mitigate abnormal signals that arise from certain types of scale defects or contamination without the need for complex signal processing would be desirable.